A light emitting diode (LED) is a semiconductor diode for radiating light, and has a function of converting an electrical energy into visible light or ultraviolet light. Particularly, for the purpose of utilizing the visible lights, there has been widely used as LED lamp in which a semiconductor light emitting element (light emitting chip) formed of light emitting materials such as GaP, GaAsP, GaAlAs or the like is sealed by a transparent resin or the like. Further, there has been also widely used as light emitting apparatuses such as display-type LED lamp or the like in which the light emitting material is fixed on an upper surface of a printed circuit board or a metal lead, and the fixed material is sealed by a resin case formed in a shape of numerical character or letter.
Further, when various phosphor powders are contained into a front surface of the semiconductor light emitting element or contained in the resin covering the light emitting diode, the light emitting apparatus capable of suitably controlling the color of the emitted lights has been also in practical use. That is, there has been known also a light emitting apparatus comprising: a gallium nitride type light emitting diode for emitting blue lights or ultraviolet lights; and a phosphor excited by the blue lights or the ultraviolet lights for emitting visible lights having a different wavelength, so that a white light is generated by an appropriate color mixing operation.
In this light emitting apparatus, for example, there has been used a yttrium.aluminum.garnet (YAG) type phosphor having a composition of [(R, Sm)3(Al, Ga)5O12] or the like. In the above light emitting apparatus, the light emitting diode chip is fixed into an appropriate cup, and slurry prepared by blending a predetermined phosphor powder and a suitable resin is injected into the cup, followed by drying the injected material, thereby to form a phosphor layer.
FIG. 4 is a cross sectional view showing another example of a structure of a conventional light emitting apparatus comprising: the above semiconductor light emitting element (light emitting diode chip); and phosphor powders dispersed around the light emitting diode chip through a resin material. This light emitting apparatus is a surface-mounting type light emitting apparatus (SMD: Surface Mount Device) which is suitable for performing a high density package onto a mounting board.
The above semiconductor light emitting element (LED chip) 2 is adhered onto a first lead 3 by means of an adhesive agent 4 or the like. A first electrode provided onto an upper surface of the semiconductor light emitting element 2 is connected to a first lead 3 by a bonding wire 5. Further, a second electrode provided onto the upper surface of the semiconductor light emitting element 2 is connected to a second lead 7 by a bonding wire 6. Both the first and second leads 3 and 7 are connected onto a lead frame in advance. In this state, the first and second leads 3 and 7 are embedded by an injection molding method using a thermoplastic resin 8, thereby to be fixed. On to the first and second leads 3 and 7 are provided with the thermoplastic resin 8 for forming a concave portion into which the semiconductor light emitting element 2 is accommodated.
As a material for constituting the semiconductor light emitting element 2, for example, GaN type material or InGaAlP type material is used. For example, an ultraviolet light having a waveband of 330-400 nm is emitted from the GaN type light emitting element, while visible light having a waveband of 400-700 nm is emitted from the InGaAlP type light emitting element. The semiconductor light emitting element 2 is sealed by a transparent resin, and phosphor grains 10 are dispersed in the transparent resin 9. A primary light emitted from the semiconductor light emitting element 2 is absorbed by the phosphor grains 10 and subjected to a wavelength conversion thereby to be emitted as a secondary light such as white light or the like.
In this regard, the light emitting diode lamp and the light emitting apparatus can reproduce various lights in visible light region having color region ranging from blue to red which is suitably determined for concrete use. Furthermore, since the light emitting diode (LED) is a semiconductor element, LED has a long life property and a high reliability, so that an exchanging frequency of LED due to failure is effectively decreased when the LED is used as a light source. Therefore, LED has been widely used as a constituting part of mobile communication devices, personal computer peripheral equipments, OA (office automation) devices, household electrical appliances, audiovisual apparatuses, various switches, various display devices such as light source display plate for backlight or the like. Patent Document 1: Japanese Patent (Publication) No. HEI 10-215001
However, since the powdery phosphor used in the above conventional light emitting apparatus was opaque, the light emitted from an inner portion of the phosphor layer formed by piling up the phosphor powders was disadvantageously absorbed and scattered by the opaque phosphors located at outside the outstanding phosphor. As a result, it was difficult to effectively extract the emitted light to outside the light emitting element.
In order to cope with the above situation, in a case where a thickness of the phosphor layer is decreased to be thin for the purpose of increasing the light extraction efficiency of extracting the light emitted from the phosphor toward outside, among a total amount of the lights emitted from the semiconductor light emitting element, an amount of light which is not effectively absorbed in the phosphor and uselessly emitted to outside of the light emitting element is increased, thereby to lower an entire light extracting efficiency of the light emitting apparatus.
On the other hand, in a case where the thickness of the phosphor layer is increased to be thick for the purpose of allowing the phosphor layer to sufficiently absorb the light emitted from the semiconductor light emitting element, the light extraction efficiency of extracting the light emitted from the phosphor is also disadvantageously lowered, so that there had been posed a problem of lowering the entire light extracting efficiency of the light emitting apparatus.
Further, in a case where a semiconductor light emitting element for emitting ultraviolet light is used, the resin mixed with the phosphor powder was liable to be deteriorated by the ultraviolet light, so that there had been posed a problem that a life duration of the light emitting apparatus was disadvantageously limited and the durability of the light emitting apparatus was also lowered. Furthermore, in production lines of a light emitting apparatus formed by using a dip method, there was a difficulty of that a processing time required until the resin was cured was long, so that a mass-productivity of the light emitting apparatus was disadvantageously lowered.
Further, in the conventional light emitting diode apparatus, a color tone of the emitted light is determined by a concentration and an injection amount of the phosphor powder to be contained in the resin. However, a difference in specific gravity between the resin and the phosphor powder is large, so that a management of the concentration of the phosphor is difficult. As a result, there has been also posed a problem that the concentration of the phosphor becomes uneven for the respective operations of injecting the phosphor.
Furthermore, an injection amount of a resin suspension of the phosphor with respect to each of the light emitting apparatus is extremely small to be 0.1 mcc or less, so that it was extremely difficult to accurately control such a small amount so as to have a predetermined value in a level of mass-production process. As a result, there has been arisen a problem that the injection amount and a content of the phosphor are greatly fluctuated, so that the color tone of the light emitted from the light emitting apparatus is not uniform.
As a method of solving such problems, there has been known a countermeasure in which the phosphor was contained in a solid-state resin. In this case, however, the concentration of the phosphor is low and a sufficient emission output could not be obtained. In addition, in this case, when the concentration of the phosphor is increased, there has been arisen a problem to be technically solved such that a transparency of the resin layer containing the phosphor is lowered thereby to decrease the emission output.
The present invention has been achieved to solve the above conventional problems, and an object of the present invention is to provide a light emitting apparatus having a high light-extraction efficiency of extracting the light emitted from the phosphors and is excellent in luminescence intensity.